\section{Conclusion}
\label{sec:conclusion}

Three decades of minute-level data deliver a sharper statement of intraday
market fragility than either the efficiency or the fragility narrative alone.
Isolated idiosyncratic shocks---even six-sigma events in the largest five
hundred securities---do not propagate: their abnormal cross-sectional
footprint is statistically and economically zero at every horizon from one
minute to two hours, in every decade. What changed is the market's handling of
common variation: information that in the mid-1990s took half an hour to
appear in cross-stock comovement now arrives within the minute, ordinary
extreme moves became an order of magnitude rarer, and extreme synchronization
concentrated in systemic co-jump minutes whose frequency explodes in stress
episodes. Fragility migrated from securities to minutes.

The evidence is deliberately associational where identification runs out:
OHLCV bars do not observe quotes, depth, or order flow, so liquidity operates
through proxies; tickers are not permanent identities, so long-run statements
run within screened spells; and the co-jump series conflates common news with
endogenous synchronization---separating the two requires timestamped news and
quote data, which we leave as the natural extension. For market design and
risk monitoring, the practical implication is direct: surveillance aimed at
single-name events monitors the part of the system that modern markets already
absorb; the binding constraint is the market-wide minute.